The invention relates to a process for the low-temperature fractionation of air.
Relevant air-fractionation processes and apparatuses are described, for example, in Hausen/Linde, Tieftemperaturtechnik [Cryogenics], 2nd edition 1985, Chapter 4 (pages 281 to 337). The invention relates in particular to two-column or multicolumn systems having a pressure column and having a low-pressure column disposed above the pressure column and/or a multicolumn system having further separation columns for nitrogen-oxygen separation. The pressure column in this case is the "first rectification column" in the meaning of the invention; the rectification in the low-pressure column and/or the vaporization in the top condenser of the crude-argon column is the "further process step". The "transfer fraction" is here formed by the bottoms liquid or an intermediate liquid of the pressure column, which liquid is introduced into the low-pressure column or into the vaporization space of the top condenser of the crude-argon column.
The invention relates in particular to double-column processes, as presented in FIGS. 4.21, 4.23, 4.26, 4.28 and 4.34 in Chapter 4.5 of Hausen/Linde. As a difference from the examples in Hausen/Linde, in the invention, the mass transfer is preferably effected in at least one separation column (e.g. low-pressure column and/or crude-argon column) at least in part by a random packing or arranged packing.
The transfer fraction collects within the first rectification column in a reservoir which is formed by the bottom of this column or a receptacle situated in the column. The liquid level in this reservoir establishes the "first level" h1 in the meaning of the invention. From this reservoir, the transfer fraction is passed into a vessel in which a further process step is carried out, for example the low-pressure column or the vaporization space of a condenser-evaporator (e.g. top condenser of the crude-argon column). The position of the feed to this further process step defines the "second, higher level", in the meaning of the invention.
For some years, the use of low-pressure-drop internals in air fractionation columns has been becoming increasingly widespread, since they have a number of advantages. Air fractionation plants in which packings are used in the low-pressure part of a double column are described, for example, in EP 321163 A, WO 93319335 WO 9319336 or EP 628777 A.
A disadvantage of the use of packings is that the height increases notably compared with tray column. In this case, the inequality quoted in the patent claim can apply, that is to say the pressure difference between pressure column and low-pressure column or between pressure column and evaporation space of the top condenser of the crude-argon column is no longer sufficient in order to overcome the corresponding hydrostatic pressure of a liquid column of the transfer fraction. Whereas this situation can occur in some plants even under normal operating conditions under full load, it frequently appears in particular during special operating cases, in particular during operations under reduced load, that is at lower product and feed rate than under full load operation.
The problem has already been mentioned in principle in EP 567360 A and solved by injecting a "lift gas" downstream of the valve.
The object underlying the invention is further to improve the abovementioned process and the corresponding apparatus.
In the context of the invention it has proved that it is possible to produce the "lift gas" in the meaning of EP 567360 A directly from the transfer fraction itself. The disadvantages of the method described in BP 567360 A are avoided in this case, in particular, in the transfer of oxygen-enriched liquid from the pressure column, neither is consumption of pressurized air as "lift gas" nor are complex additional steps for producing "lift gas" from the transfer fraction necessary; an additional controller is also dispensed with.
For this, a disposition of the expansion valve on a suitable intermediate level between the first and second level is required. The specific establishment of this intermediate level is different for each specific embodiment of the invention, but it can be determined without problem using calculation tools which are available to those skilled in the art, if the height of the intermediate level is given as a degree of freedom. In typical cases, the expansion valve is at an intermediate level of EQU hz =h1+x.multidot.(h2-h1),
where x is 30 to 80%, preferably 40 to 70%.
The plant must be designed for a defined operating case. for example for starting up the plant. In another example, the disposition of the expansion valve is designed for the low-load case in steady-state operation of the plant; then, in some circumstances, additional means must be provided for transporting the transfer liquid to the "further process step" during the start-up of the plant; in this case, conventional methods for transporting liquid (mechanical pump, injection of external gas etc.) can be used, alternatively or additionally, the pressure level in the first rectification column can be increased during start-up.
In the process of the invention it is expedient if the transfer fraction is subcooled by indirect heat exchange upstream of the expansion. As a result the formation of a two phase mixture upstream of the expansion is wholly or partially avoided, so that the specific vapour bubble formation of the invention does not take place until during expansion. The subcooling is generally performed in the vicinity of the first level.
Preferably, subcooling is performed just so intensively that the transfer fraction, immediately upstream of the expansion, is completely, or essentially completely, present in liquid form, but is not subcooled further.
In the design of a plant, this is carried out in practice in such a manner that the subcooling is firstly established. The extent of the subcooling of the transfer fraction is generally determined independently of the liquid transport process and is determined by other criteria, for example the aim of producing relatively little flash gas during injection into the second vessel. The expansion operation, in particular the disposition of the expansion valve, is then determined in such a manner that during the predetermined subcooling the transfer fraction is just still present in a single-phase liquid state immediately upstream of the expansion and neither significant subcooling nor vapour bubbles are present to a significant extent.
The invention further relates to an apparatus for the low-temperature fractionation of air.